Regulated splicing of the cellular transcriptome is essential for normal growth and development. Aberrant expression of splicing patterns is linked to cancers and many other diseases. These abnormal splicing patterns may arise from mutations in pre-mRNA splice sites or in the protein components of the splicing machinery. Recent exome analysis of myelodysplastic syndrome (MDS) patients indicates a link between point mutations in core proteins of the splicing machinery and disease pathogenesis. Somatic mutations in eight splicing proteins were found to be recurrent in MDS cases. This association makes it essential to understand the mechanisms by which these spliceosomal proteins mutations lead to a disease phenotype. We hypothesize that the pathogenic effects of MDS mutations in splicing factors are mediated through changes in their splicing functions. The MDS mutations affect the normal functions of these proteins, resulting in generation of abnormal splicing patterns of transcripts that are important in hematopoiesis. This causes aberrant growth control in HSPCs. The goal of this project is to determine the molecular effects of MDS mutations on splicing factor function and to identify genome-wide splicing aberrations they induce during hematopoiesis that contribute to MDS pathogenesis. We will focus on mutations in SF3B1 and U2AF35 proteins. Distribution of mutations in these two proteins differs amongst the MDS types. SF3B1 mutations are frequent in MDS types that are characterized by ringed sideroblasts (RS), whereas U2AF35 mutations occur in forms of MDS without RS. This mutually exclusive occurrence and their very specific disease outcomes indicate differences in their pathogenic effect and likely distinct mechanisms of disease progression. We will use advanced high density RNA sequencing techniques to identify the MDS mutation induced aberrations in splicing profile of hematopoietic stem/progenitor cells. Effect of these mutations on normal proliferation and differentiation of HSPCs will be determined. Mechanisms underlying aberrant splicing will be determined by defining the MDS mutation- induced changes in U2AF35 and SF3B1 splicing functions using biochemical techniques. Using this approach we expect to define specific splicing events controlled by U2AF35 and SF3B1 during normal hematopoiesis and then elucidates how their mutation contributes to MDS pathogenesis.